1. Field of the Invention
The present invention relates to a diffraction grating used in various kinds of optical devices such as an optical pickup, a spectroscope, a demultiplexer, etc.
2. Description of the Related Art
A diffraction grating is one of general and important optical elements. Such a diffraction grating is described in e.g., "8.6. Fraunhofer's diffraction in an optical instrument" in a diffraction theory in chapter VIII of "Principle II of optics" written by M. Born and E. Wolf (a 15th edition translated by Tohru Kusagawa and Hidetsugu Yokota published from Tokai University Press in May, 1975. The diffraction grating is an element for dividing light having non-single wavelengths every a single wavelength.
In such a diffraction grating, when an incident angle of light incident to the diffraction grating is set to .theta..sub.i, an angle .theta..sub.o of diffraction, a wavelength .lambda. of the light and a grating pitch is set to .LAMBDA., a phase matching condition is generally provided as follows. EQU sin .theta.i-sin .theta..smallcircle.=m.lambda./.LAMBDA. (m=0, .+-.1, .+-.2, . . . ) (1)
A change in angle of the light caused by a change in wavelength is called angular dispersion. From the above formula (1), this angular dispersion is represented by the following formula (2). EQU d.theta./d.lambda.=m/.LAMBDA. cos .theta. (2)
In accordance with this formula (2), the angular dispersion is inversely proportional to the grating pitch .LAMBDA.. Therefore, to reduce the angular dispersion, it is necessary to increase the grating pitch.
However, when the grating pitch is increased in low angular dispersion, diffraction efficiency is reduced. Namely, to operate the diffraction grating of this kind with high efficiency, it is necessary to cause Bragg diffraction by reducing the grating pitch to a length about the wavelength of light. Therefore, at the present time, it is difficult to consist the low angular dispersion and the high diffraction efficiency with each other so that such a diffraction grating is practically used only in a limited range.